Heretofore, non-reclaimable thermoplastic polymeric scrap materials have, for the most part, been disposed of as land fill. As the availability of land fill sites diminishes, the cost of plastic waste disposal in this manner increases.
Polymer waste is generated in many ways. For example, a great amount of thermoplastic paint sludge is collected from paint spray booths. Overspray is first flocculated in a water cascade and then collected as sludge. The sludge is dried and disposed of in sealed containers. Other sources of polymer waste are pure or highly filled thermoplastic injection or compression molded compositions. Molding materials with both thermoplastic and thermosetting constituents are common because such materials cannot be readily reprocessed.
It is well known that most polymers can be burned, and that the burning reaction produces a substantial amount of heat energy. Thus, incineration has been considered as an alternative to solid waste disposal. Incinerating thermoplastics, however, presents a number of serious problems. For example, in conventional incinerators, thermoplastic polymers have a tendency to melt. The molten material inhibits uniform combustion which may cause excess smoke production, incomplete incineration and potentially explosive gases. Moreover, thermoplastic waste materials have a wide range of heating values. For example, a paint sludge having a high heating value might produce elevated temperatures that could damage a conventional incinerator. If the heating value of a paint sludge is low, the sludge may not burn continuously or completely.
In my search for alternative methods of burning polymer scrap, consideration was given to the use of fluidized bed incinerators. However, conventional fluidized bed incinerators used to burn coal, paper, wood and other such materials are not suitable for incinerating all polymers, particularly thermoplastics. Thermoplastics melt and clog the fluidized bed. Moreover, conventional feeding systems for coal and other nonmeltable fuels are not adaptable to feeding meltable plastic scrap materials. Further, in conventional fluidized beds the burning reaction of thermoplastics tends to be self extinguishing, i.e., combustion with oxygen cannot be fully sustained within the bed. This may lead to a high concentration of volatile polymer by-products within the reactor and incumbent danger of explosion.
By way of definition, the terms incineration and burning herein refer to the thermal degradation of reaction of polymers in the presence of enough oxygen to support combustion of all burnable constituents at suitable elevated temperatures. Pyrolysis refers to the degradation reaction of such polymers at elevated temperatures in an oxygen deficient atmosphere.